.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
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.\" $Id: cmsg.3,v 1.8 2000/12/20 18:10:31 ak Exp $
.TH CMSG 3 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
CMSG_ALIGN, CMSG_SPACE, CMSG_NXTHDR, CMSG_FIRSTHDR \- access ancillary data
.SH SYNOPSIS
.nf
.B #include <sys/socket.h>
.PP
.BI "struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *" msgh );
.BI "struct cmsghdr *CMSG_NXTHDR(struct msghdr *" msgh ,
.BR "                            struct cmsghdr *" cmsg );
.BI "size_t CMSG_ALIGN(size_t " length );
.BI "size_t CMSG_SPACE(size_t " length );
.BI "size_t CMSG_LEN(size_t " length );
.BI "unsigned char *CMSG_DATA(struct cmsghdr *" cmsg );
.fi
.SH DESCRIPTION
These macros are used to create and access control messages (also called
ancillary data) that are not a part of the socket payload.
This control information may
include the interface the packet was received on, various rarely used header
fields, an extended error description, a set of file descriptors, or UNIX
credentials.
For instance, control messages can be used to send
additional header fields such as IP options.
Ancillary data is sent by calling
.BR sendmsg (2)
and received by calling
.BR recvmsg (2).
See their manual pages for more information.
.PP
Ancillary data is a sequence of
.I cmsghdr
structures with appended data.
See the specific protocol man pages for the available control message types.
The maximum ancillary buffer size allowed per socket can be set using
.IR /proc/sys/net/core/optmem_max ;
see
.BR socket (7).
.PP
The
.I cmsghdr
structure is defined as follows:
.PP
.in +4n
.EX
struct cmsghdr {
    size_t cmsg_len;    /* Data byte count, including header
                           (type is socklen_t in POSIX) */
    int    cmsg_level;  /* Originating protocol */
    int    cmsg_type;   /* Protocol\-specific type */
/* followed by
   unsigned char cmsg_data[]; */
};
.EE
.in
.PP
The sequence of
.I cmsghdr
structures should never be accessed directly.
Instead, use only the following macros:
.IP * 3
.BR CMSG_FIRSTHDR ()
returns a pointer to the first
.I cmsghdr
in the ancillary
data buffer associated with the passed
.IR msghdr .
It returns NULL if there isn't enough space for a
.I cmsghdr
in the buffer.
.IP *
.BR CMSG_NXTHDR ()
returns the next valid
.I cmsghdr
after the passed
.IR cmsghdr .
It returns NULL when there isn't enough space left in the buffer.
.IP
When initializing a buffer that will contain a series of
.I cmsghdr
structures (e.g., to be sent with
.BR sendmsg (2)),
that buffer should first be zero-initialized
to ensure the correct operation of
.BR CMSG_NXTHDR ().
.IP *
.BR CMSG_ALIGN (),
given a length, returns it including the required alignment.
This is a
constant expression.
.IP *
.BR CMSG_SPACE ()
returns the number of bytes an ancillary element with payload of the
passed data length occupies.
This is a constant expression.
.IP *
.BR CMSG_DATA ()
returns a pointer to the data portion of a
.IR cmsghdr .
The pointer returned cannot be assumed to be suitably aligned for
accessing arbitrary payload data types.
Applications should not cast it to a pointer type matching the payload,
but should instead use
.BR memcpy (3)
to copy data to or from a suitably declared object.
.IP *
.BR CMSG_LEN ()
returns the value to store in the
.I cmsg_len
member of the
.I cmsghdr
structure, taking into account any necessary
alignment.
It takes the data length as an argument.
This is a constant
expression.
.PP
To create ancillary data, first initialize the
.I msg_controllen
member of the
.I msghdr
with the length of the control message buffer.
Use
.BR CMSG_FIRSTHDR ()
on the
.I msghdr
to get the first control message and
.BR CMSG_NXTHDR ()
to get all subsequent ones.
In each control message, initialize
.I cmsg_len
(with
.BR CMSG_LEN ()),
the other
.I cmsghdr
header fields, and the data portion using
.BR CMSG_DATA ().
Finally, the
.I msg_controllen
field of the
.I msghdr
should be set to the sum of the
.BR CMSG_SPACE ()
of the length of
all control messages in the buffer.
For more information on the
.IR msghdr ,
see
.BR recvmsg (2).
.SH CONFORMING TO
This ancillary data model conforms to the POSIX.1g draft, 4.4BSD-Lite,
the IPv6 advanced API described in RFC\ 2292 and SUSv2.
.BR CMSG_FIRSTHDR (),
.BR CMSG_NXTHDR (),
and
.BR CMSG_DATA ()
are specified in POSIX.1-2008.
.BR CMSG_SPACE ()
and
.BR CMSG_LEN ()
.\" https://www.austingroupbugs.net/view.php?id=978#c3242
will be included in the next POSIX release (Issue 8).
.PP
.BR CMSG_ALIGN ()
is a Linux extension.
.SH NOTES
For portability, ancillary data should be accessed using only the macros
described here.
.BR CMSG_ALIGN ()
is a Linux extension and should not be used in portable programs.
.PP
In Linux,
.BR CMSG_LEN (),
.BR CMSG_DATA (),
and
.BR CMSG_ALIGN ()
are constant expressions (assuming their argument is constant),
meaning that these values can be used to declare the size of global variables.
This may not be portable, however.
.SH EXAMPLES
This code looks for the
.B IP_TTL
option in a received ancillary buffer:
.PP
.in +4n
.EX
struct msghdr msgh;
struct cmsghdr *cmsg;
int received_ttl;

/* Receive auxiliary data in msgh */

for (cmsg = CMSG_FIRSTHDR(&msgh); cmsg != NULL;
        cmsg = CMSG_NXTHDR(&msgh, cmsg)) {
    if (cmsg\->cmsg_level == IPPROTO_IP
            && cmsg\->cmsg_type == IP_TTL) {
        memcpy(&receive_ttl, CMSG_DATA(cmsg), sizeof(received_ttl));
        break;
    }
}

if (cmsg == NULL) {
    /* Error: IP_TTL not enabled or small buffer or I/O error */
}
.EE
.in
.PP
The code below passes an array of file descriptors over a
UNIX domain socket using
.BR SCM_RIGHTS :
.PP
.in +4n
.EX
struct msghdr msg = { 0 };
struct cmsghdr *cmsg;
int myfds[NUM_FD];  /* Contains the file descriptors to pass */
char iobuf[1];
struct iovec io = {
    .iov_base = iobuf,
    .iov_len = sizeof(iobuf)
};
union {         /* Ancillary data buffer, wrapped in a union
                   in order to ensure it is suitably aligned */
    char buf[CMSG_SPACE(sizeof(myfds))];
    struct cmsghdr align;
} u;

msg.msg_iov = &io;
msg.msg_iovlen = 1;
msg.msg_control = u.buf;
msg.msg_controllen = sizeof(u.buf);
cmsg = CMSG_FIRSTHDR(&msg);
cmsg\->cmsg_level = SOL_SOCKET;
cmsg\->cmsg_type = SCM_RIGHTS;
cmsg\->cmsg_len = CMSG_LEN(sizeof(myfds));
memcpy(CMSG_DATA(cmsg), myfds, sizeof(myfds));
.EE
.in
.PP
For a complete code example that shows passing of file descriptors
over a UNIX domain socket, see
.BR seccomp_unotify (2).
.SH SEE ALSO
.BR recvmsg (2),
.BR sendmsg (2)
.PP
RFC\ 2292
.SH COLOPHON
This page is part of release 5.13 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.
